1. Field of the Invention
This invention pertains to active optical sensing systems, and more particularly, to ladars and diffuse optical tomography.
2. Description of Related Art
There are at least two types of active optical sensing systems: ladars and diffuse optical tomography systems. A typical ladar system includes a light source, which is a transmitter, and a detector, which is a receiver. The transmitter can be a coherent or incoherent source. The receiver can be a simple detector or a performance-enhanced detector. Some ladars are designed to measure the spectral properties of a subject. The transmitter is a wavelength-tunable laser or multiple lasers of different wavelengths. By comparing the relative absorption by the target at different wavelengths, the system can infer the medium properties. In a wavelength-tunable single-frequency laser transmitter, only one wavelength can be used at a time; the system is a time-division multiplexing system. In a system with many lasers of different wavelength, the signal of each wavelength must be differentiated or discriminated from the others, and the method for discrimination can be time division multiplexing or frequency division multiplexing, in which each laser is modulated with a unique frequency. The receiver distinguishes laser signals by correlating a signal to a specific time slot in time division multiplexing or to a specific known frequency in frequency division multiplexing. The drawback is low efficiency and high cost.
Ladar systems are disclosed in the following U.S. published patent applications, the disclosures of which are incorporated into this patent by this reference:                20030137647 Hybrid optical correlator/digital processor for target detection and discrimination        20030076485 Ladar system for detecting objects        20030067537 System and method for three-dimensional data acquisition        20030052169 Planar laser illumination and imaging (PLIIM) based camera system for producing high-resolution 3-D images of moving 3-D objects        20020131163 Optical amplifier employing an active doped unitary amplifier        
In a typical diffuse optical tomography system, the function of a system is to determine the spatial profile of the absorption and scattering coefficients of a medium, and reconstruct an image of the medium. The prior art approach is to have a single source location at a time, and all receivers measure the signal in parallel. Continuous wave, frequency domain, and time domain methods are known and used in the prior art for signal modulation and measurement. In each of the three methods, the transmitter is a single laser of one wavelength, or multiple lasers of different wavelengths. An essential aspect of diffuse optical tomography is the location of the transmitter and receiver relative to the subject of interest. If the transmitter needs to move from one location to another, the prior art method is to optically switch the transmitter light beam from one port to another port that leads the beam to another location. If the transmitter needs to change the wavelength, the input laser must be tuned from one wavelength to another. When multiple lasers of different wavelengths are used, the lasers must take turns to be turned on and off, in order to change the transmitter output wavelength. The transmitter is operated in the time-division-multiplexing mode. The drawback is that the switching time is limited by the speed of the optical switches employed. Diffuse optical tomography and other optical systems can be found in U.S. Pat. Nos. 6,577,884; 6,516,214; 5,853,370; 5,353,799; 5,421,329; 5,282,467; 5,782,237; 5,553,614; 5,792,051; 5,902,235; 5,795,292; 5,697,367; 5,584,296; 5,482,034; 5,477,853; 5,465,714; 5,217,013; 5,140,989; 5,139,025; 4,817,623; 4,768,516; 4,725,147; 4,570,638; and 5,779,631, the disclosures of which are incorporated into this patent by this reference.
Opto-electronic systems and methods, and related devices, for remote detection of physical magnitudes, are disclosed in the following U.S. patents, the disclosures of which are incorporated into this patent by this reference:                U.S. Pat. No. 5,774,215 Reduced complexity fourier transform spectrometer        U.S. Pat. No. 5,564,832 Birefringent active fiber laser sensor        U.S. Pat. No. 5,513,913 Active multipoint fiber laser sensor        U.S. Pat. No. 5,381,010 Periodically alternating path and alternating wavelength bridges for quantitative and ultrasensitive measurement of vapor concentration        U.S. Pat. No. 5,307,146 Dual-wavelength photometer and fiber optic sensor probe        U.S. Pat. No. 5,299,869 Laser diode temperature sensing system        U.S. Pat. No. 5,200,796 Method and apparatus for detecting and measuring a physical magnitude        U.S. Pat. No. 5,191,458 Optical electronic multiplexing reflection sensor system        U.S. Pat. No. 5,179,424 Optoelectronic apparatus for the remote measuring of a physical magnitude        U.S. Pat. No. 4,814,604 Opto-electronic method and system for remote detection of physical magnitudes        
Many optical sensing systems require multiple detections of several properties of the target. Using the existing techniques often results in high cost from time-consuming optical sensing. What is needed is a robust, highly efficient, and low cost method capable of simultaneous detection of target properties.